0001628280-23-015763 | 10-Q | XBRL Viewer

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Cover Page - shares	3 Months Ended
Cover Page - shares	Mar. 31, 2023	May 01, 2023
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2023
Document Transition Report	false
Entity File Number	001-38295
Entity Registrant Name	X4 PHARMACEUTICALS, INC
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	27-3181608
Entity Address, Address Line One	61 North Beacon Street
Entity Address, Address Line Two	4th Floor
Entity Address, City or Town	Boston
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02134
City Area Code	857
Local Phone Number	529-8300
Title of 12(b) Security	Common Stock
Trading Symbol	XFOR
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		122,207,488
Amendment Flag	false
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	Q3
Entity Central Index Key	0001501697
Current Fiscal Year End Date	--12-31

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 93,406	$ 121,718
Research and development incentive receivable	658	1,152
Prepaid expenses and other current assets	4,158	5,807
Total current assets	98,222	128,677
Property and equipment, net	986	1,104
Goodwill	17,351	17,351
Right-of-use assets	6,844	7,229
Other assets	1,002	1,225
Total assets	124,405	155,586
Current liabilities:
Accounts payable	5,992	7,777
Accrued expenses	11,762	12,034
Current portion of lease liability	1,175	1,198
Current portion of long-term debt	554	1,315
Total current liabilities	19,483	22,324
Long-term debt, net of discount and current portion	31,827	32,304
Lease liabilities	3,377	3,603
Warrant liability (Note 4)	17,692	23,131
Other liabilities	350	173
Total liabilities	72,729	81,535
Commitments and contingencies (Note 9)
Stockholders’ equity:
Common stock, $0.001 par value, 500,000,000 shares authorized as of March 31, 2023 and December 31, 2022, respectively; 122,207,488 and 121,667,250 shares issued and outstanding as of March 31, 2023 and December 31, 2022, respectively	122	122
Additional paid-in capital	452,431	450,786
Accumulated other comprehensive loss	(119)	(119)
Accumulated deficit	(400,758)	(376,738)
Total stockholders’ equity	51,676	74,051
Total liabilities and stockholders’ equity	$ 124,405	$ 155,586

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Mar. 31, 2023	Dec. 31, 2022
Statement of Financial Position [Abstract]
Common stock, par value (in usd per share)	$ 0.001	$ 0.001
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	122,207,488	121,667,250
Common stock, shares outstanding	122,207,488	121,667,250

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Operating expenses:
Research and development	$ 22,063	$ 14,113
Selling, general and administrative	7,241	7,664
Gain (Loss) on Disposition of Assets	0	(509)
Total operating expenses	29,304	21,268
Loss from operations	(29,304)	(21,268)
Other income (expense), net:
Interest income	835	3
Interest expense	(1,109)	(913)
Change in fair value of derivative liability	5,439	176
Other income, net	123	60
Total other income (expense), net	5,288	(674)
Loss before provision for income taxes	(24,016)	(21,942)
Provision for income taxes	4	23
Net loss and comprehensive loss	(24,020)	(21,965)
Deemed dividend on Class B Warrant price reset	0	(2,259)
Net loss attributable to common stockholders	$ (24,020)	$ (24,224)
Net loss per share attributable to common stockholders—basic	$ (0.16)	$ (0.72)
Net loss per share attributable to common stockholders—diluted	$ (0.16)	$ (0.72)
Weighted average shares of common stock outstanding—diluted	145,967,000	33,737,000
Weighted average shares of common stock outstanding—basic	145,967,000	33,737,000

CONDENSED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK, REDEEMABLE COMMON STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]
Beginning balance, shares at Dec. 31, 2021		28,127,657
Beginning balance at Dec. 31, 2021	$ 64,412	$ 28	$ 347,374	$ (119)	$ (282,871)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Shares, New Issues		2,512,902
Stock Issued During Period, Value, New Issues	5,820	$ 3	5,817
Restricted Stock, Shares Issued Net of Shares for Tax Withholdings		168,817
Restricted Stock, Value, Shares Issued Net of Tax Withholdings	(12)		(12)
Stock Issued During Period Shares Warrants Exercised		100
APIC, Share-Based Payment Arrangement, Increase for Cost Recognition	1,459		1,459
Net loss	(21,965)				(21,965)
Ending balance, shares at Mar. 31, 2022		30,809,476
Ending balance at Mar. 31, 2022	49,714	$ 31	354,638	(119)	(304,836)
Beginning balance, shares at Dec. 31, 2022		121,667,250
Beginning balance at Dec. 31, 2022	74,051	$ 122	450,786	(119)	(376,738)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Shares, New Issues		540,238
APIC, Share-Based Payment Arrangement, Increase for Cost Recognition	1,645		1,645
Net loss	(24,020)				(24,020)
Ending balance, shares at Mar. 31, 2023		122,207,488
Ending balance at Mar. 31, 2023	$ 51,676	$ 122	$ 452,431	$ (119)	$ (400,758)

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)
$ in Thousands

3 Months Ended

Mar. 31, 2023

Mar. 31, 2022

Cash flows from operating activities:

Net loss

$ (24,020)

$ (21,965)

Adjustments to reconcile net loss to net cash used in operating activities:

Stock-based compensation expense

1,645

1,459

Depreciation and amortization expense

127

133

Non-cash lease expense

385

366

Accretion of debt discount

225

198

Change in fair value of warrant liability

(5,439)

Other

(51)

(120)

Changes in operating assets and liabilities:

Prepaid expenses, other current assets and research and development incentive receivable

2,084

1,633

Accounts payable

(1,714)

(713)

Accrued expenses

496

(984)

Lease liabilities

(250)

(236)

Net cash used in operating activities

(26,512)

(20,229)

Cash flows from investing activities:

Acquisition of property and equipment

(9)

(22)

Net cash used in investing activities

(9)

(22)

Cash flows from financing activities:

Employee taxes paid related to net share settlement of vested restricted stock units

(12)

Fees paid to amendment loan and security agreement and issuance costs related to the sale of warrants

(381)

(114)

Repayments of Secured Debt

(1,300)

(795)

Proceeds from sale of shares of common stock, and pre-funded warrants, net of issuance costs (1)

^[1]

5,877

Payments for Repurchase of Common Stock

^[1]

(443)

Net cash (used in) provided by financing activities

(2,124)

4,956

Effect of exchange rate changes on cash, cash equivalents and restricted cash

(69)

Net decrease in cash, cash equivalents and restricted cash

(28,595)

(15,364)

Cash, cash equivalents and restricted cash at beginning of period

123,028

83,108

Cash, cash equivalents and restricted cash at end of period

$ 94,433

$ 67,744

[1]	(1) For the three month period ended March 31, 2023, includes payment of issuance costs related to December 2022 public sale of shares of common stock and pre-funded warrants

CONDENSED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK, REDEEMABLE COMMON STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Parenthetical) $ in Thousands	3 Months Ended
	Dec. 31, 2022 USD ($)
Statement of Stockholders' Equity [Abstract]
Issuance costs

Nature of the Business and Basis of Presentation	3 Months Ended
Nature of the Business and Basis of Presentation	Mar. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	NATURE OF THE BUSINESS AND BASIS OF PRESENTATION X4 Pharmaceuticals, Inc. (together with its subsidiaries, the “Company”) is a late-stage clinical biopharmaceutical company focused on the research and development of novel therapeutics for the treatment of rare diseases and those with limited treatment options, with a focus on conditions resulting from dysfunction of the immune system. The Company’s lead clinical candidate is mavorixafor, a small molecule antagonist of the chemokine receptor CXCR4 that is being developed as an oral, once-daily therapy. Due to its ability to increase the mobilization of mature, functional white blood cells from the bone marrow into the bloodstream, the Company believes that mavorixafor has the potential to provide therapeutic benefit across a variety of chronic neutropenic disorders, including WHIM (Warts, Hypogammaglobulinemia, Infections, and Myelokathexis (“WHIM”)) syndrome, a rare, primary immunodeficiency, and certain cancers. Following announcement of positive top-line data from the Company’s global, pivotal, Phase 3 clinical trial in November 2022, the Company is preparing a United States regulatory submission seeking approval of oral, once-daily mavorixafor in the treatment of people aged 12 years and older with WHIM syndrome. The Company is also currently enrolling participants in a Phase 2 clinical trial in people with certain chronic neutropenic disorders following positive results from a Phase 1b clinical trial evaluating mavorixafor in people with idiopathic, cyclic, or congenital neutropenia that were presented in the third quarter of 2022. The Company also conducted a proof-of-concept Phase 1b clinical trial of mavorixafor in combination with ibrutinib in people with Waldenström’s macroglobulinemia (“Waldenström’s”), a rare form of lymphoma. The Company reported positive results from the Waldenström’s Phase 1b trial in the third quarter of 2022 and concluded the trial in December 2022. Any further studies of mavorixafor in Waldenström’s or other oncology indication will be subject to completing a strategic partnership. The Company is headquartered in Boston, Massachusetts and has an additional facility in Vienna, Austria. Going Concern Assessment—In accordance with Accounting Standards Update (“ASU”) No. 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (Subtopic 205-40) (“ASU 2014-15”), the Company has evaluated whether there are certain conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the condensed consolidated financial statements are issued. Since inception, the Company has incurred significant operating losses and negative cash flows from operations. As of March 31, 2023, the Company had $93.4 million of cash and cash equivalents, an accumulated deficit of $400.8 million and net cash used in operating activities of $26.5 million. The Company has a covenant under its Second Amended and Restated Loan and Security Agreement with Hercules Capital Inc. (“Hercules”) that requires that the Company maintain a minimum level of cash of $20 million, subject to reduction to $10 million upon the achievement of operational milestones. Based on its current cash flow projections and with no additional funding, the Company believes it will not be able to maintain the minimum cash required to satisfy this covenant beginning in the first quarter of 2024. In such event, the lender could require the repayment of all outstanding debt. Management has assessed the Company’s ability to continue as a going concern in accordance with the requirements of ASC 205-40 and determined that the Company’s accumulated deficit, history of losses and future expected losses met the ASC 205-40 standard for raising substantial doubt about the Company’s ability to continue as a going concern. The Company does not have adequate financial resources to fund its forecasted operating costs for at least one year after the date that these condensed consolidated financial statements are issued. The accompanying condensed consolidated financial statements do not include any adjustments relating to the recoverability and classification of recorded asset amounts or the amounts and classification of liabilities that might result from the outcome of this uncertainty. Accordingly, the condensed consolidated financial statements have been prepared on a basis that assumes the Company will continue as a going concern, which contemplates the realization of assets and satisfaction of liabilities and commitments in the ordinary course of business. To finance its operations, the Company will need to raise additional capital, which cannot be assured. Unless and until the Company reach’s profitability in the future, it will require additional capital to fund our operations, which could be raised through a combination of equity offerings, debt financings, other third-party funding, marketing and distribution arrangements and other collaborations and strategic alliances. If the Company is unable to obtain funding, it could be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which would adversely affect its business prospects, or it may be unable to continue operations. Principles of Consolidation—The condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, including X4 Pharmaceuticals (Austria) GmbH, which is incorporated in Vienna, Austria (“X4 Austria”), and X4 Therapeutics, Inc. All significant intercompany accounts and transactions have been eliminated. Unaudited Interim Condensed Consolidated Financial Statements— The condensed consolidated balance sheet at December 31, 2022 that is presented in these interim condensed consolidated financial statements was derived from audited financial statements but does not include all disclosures required by accounting principles generally accepted in the United States of America (“GAAP”). The accompanying condensed consolidated financial statements are unaudited. The accompanying unaudited interim condensed consolidated financial statements have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. Accordingly, certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These unaudited interim condensed consolidated financial statements should be read in conjunction with the Company’s audited financial statements and the notes thereto for the year ended December 31, 2022 included in the 2022 Annual Report filed with the SEC on March 21, 2023. In the opinion of management, all adjustments, consisting only of normal recurring adjustments as necessary, for the fair statement of the Company’s condensed financial position, condensed results of its operations and cash flows have been made. The results of operations for the three months ended March 31, 2023 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2023. Use of Estimates— The preparation of the Company’s consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses, the impairment or lack of impairment of long-lived assets including operating lease right-of-use assets and goodwill, and assumptions underlying the fair value of warrant liabilities. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates when there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. As of the date of issuance of these consolidated financial statements, the Company is not aware of any specific event or circumstance that would require the Company to update its estimates, assumptions and judgments or revise the carrying value of its assets or liabilities. Actual results could differ from those estimates, and any such differences may be material to the Company’s consolidated financial statements.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2023

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Significant Accounting Policies—The Company’s significant accounting policies are disclosed in the audited consolidated financial statements and the notes thereto in the Company’s Annual Report on Form 10-K for the year ended December 31, 2022 filed with the SEC on March 21, 2023. Since the date of those consolidated financial statements, there have been no material changes to the Company’s significant accounting policies.

Cash and Cash Equivalents— The Company considers all highly liquid investments with maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents consisted of money market funds as of March 31, 2023 and December 31, 2022.

Restricted Cash


(in thousands)	As of March 31, 2023		As of December 31, 2022

Letter of credit security: Waltham lease	$	250	$	250
Letter of credit security: Vienna Austria lease	206		205
Letter of credit security: Boston lease	571		855
Total restricted cash	$	1,027	$	1,310
Restricted cash included in prepaid expenses and other current assets	$	250	$	285
Restricted cash included in other assets	$	777	$	1,025

In connection with the Company’s lease agreements for its facilities in Massachusetts and Austria, the Company maintains letters of credit, which are secured by restricted cash, for the benefit of the respective landlord.

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheets to the sum of the total of amounts shown in the Company’s condensed consolidated statements of cash flows as of March 31, 2023 and December 31, 2022:


(in thousands)	March 31, 2023		December 31, 2022
Cash and cash equivalents	$	93,406	$	121,718
Restricted cash, current portion	250		285
Restricted cash, non-current	777		1,025
Total cash, cash equivalents and restricted cash	$	94,433	$	123,028

Goodwill— Goodwill is tested for impairment at the reporting unit level annually in the fourth quarter, or more frequently when events or changes in circumstances indicate that the asset might be impaired. Examples of such events or circumstances include, but are not limited to, a significant adverse change in legal or business climate, an adverse regulatory action or unanticipated competition. The Company has determined that it operates in a single operating segment and has a single reporting unit.

The Company assesses qualitative factors to determine whether the existence of events or circumstances would indicate that it is more likely than not that the fair value of the reporting unit is less than its carrying amount. If after assessing the totality of events or circumstances, the Company were to determine that it is more likely than not that the fair value of the reporting unit is less than its carrying amount, then the Company would perform an interim quantitative impairment test, whereby the Company compares the fair value of the reporting unit to its carrying value. If the fair value of the reporting unit exceeds the carrying value of its net assets, goodwill is not impaired, and no further testing is required. If the fair value of the reporting unit is less than its carrying value, the Company measures the amount of impairment loss, if any, as the excess of the carrying value over the fair value of the reporting unit. There were no triggering events during the three months ended March 31, 2023 that necessitated an interim impairment test of goodwill.

Recently Adopted Accounting Standards

In June 2016, the Financial Accounting Standards Board issued ASU 2016-13, Credit Losses (Topic 326) Measurement of Credit Losses on Financial Instruments (“ASU 2016-13”), as amended. ASU 2016-13 requires that financial assets measured at amortized cost, such as trade receivables, be presented net of expected credit losses, which may be estimated based on relevant information such as historical experience, current conditions, and future expectation for each pool of similar financial asset. The new guidance requires enhanced disclosures related to trade receivables and associated credit losses. The Company adopted ASU 2016-13 on January 1, 2023. As the Company did not have accounts receivable on its consolidated balance sheet as of the date of adoption, there was no impact to the adoption of ASU 2016-13.

License, Collaboration, and Funding Agreements	3 Months Ended
License, Collaboration, and Funding Agreements	Mar. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
License, Collaboration, and Funding Agreements	LICENSE, COLLABORATION AND FUNDING AGREEMENTS Research and Development Incentive Program The Company participates in a research and development incentive program provided by the Austrian government whereby the Company is entitled to reimbursement by the Austrian government for a percentage of qualifying research and development expenses and capital expenditures incurred by the Company’s subsidiary in Austria. As of March 31, 2023, the amount due under the program is $0.7 million, which amount is included in research and development incentive receivable in the condensed consolidated balance sheet. During the three months ended March 31, 2023 and 2022, the Company recorded $121.0 thousand and $135.3 thousand, respectively, of income related to the program within the condensed consolidated statements of operations as other income. License and Collaboration Agreements In July 2014, the Company entered into a license agreement with Genzyme (the “Genzyme Agreement”) pursuant to which the Company was granted an exclusive license to certain patents and intellectual property owned or controlled by Genzyme related to the CXCR4 receptor to develop and commercialize products containing licensed compounds (including but not limited to mavorixafor) for all therapeutic, prophylactic and diagnostic uses, with the exception of autologous and allogenic human stem cell therapy. Under the terms of the Genzyme Agreement, the Company is obligated to use commercially reasonable efforts to develop and commercialize licensed products for use in the field in the United States and at least one other major market country. The Company is obligated to pay Genzyme milestone payments in the aggregate amount of up to $25.0 million, contingent upon the achievement by the Company of certain clinical-stage regulatory and sales milestones with respect to licensed products. During three months ended March 31, 2023, the Company accrued $5.0 million related to a development milestone under the Genzyme Agreement as the Company believes that it is probable under ASC Topic 450, Contingencies, that the milestone will be achieved. The $5.0 million accrued payment has been recorded within research and development expense on the condensed consolidated statements of operations. An additional $7.0 million of regulatory milestone payments are not yet probable but are reasonably possible of becoming payable with the next twelve to eighteen month period under the Genzyme Agreement. The Company is also obligated to pay Genzyme tiered royalties based on net sales of licensed products that the Company commercializes under the agreement. Gain on Sale of Non-Financial Asset During the three months ended March 31, 2022, a third party, who had previously acquired rights to certain intellectual property from the Company, terminated the arrangement and transferred these rights back to the Company. Also during the three months ended March 31, 2022, the Company transferred these rights to another third party in return for $0.5 million. The Company has no continuing involvement in any ongoing research and development activities associated with the intellectual property. The Company concluded that these third parties are "non-customers" as the underlying intellectual property transferred to and from these third parties supports potential drug candidates that are not aligned with the Company's strategic focus and, therefore, are not an output of the Company's ordinary activities. Accordingly, the Company accounted for the sale of the intellectual property as the sale of a non-financial asset under ASC Topic 610-20, Gains and Losses from the Derecognition of Nonfinancial Assets ("ASC 610-20"), and included the gain in gain on sale of non-financial asset for the three months ended March 31, 2022. There were no material modifications of the Company’s license or collaboration agreements during the three months ended March 31, 2023.

Fair Value of Financial Assets and Liabilities

3 Months Ended

Mar. 31, 2023

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets and Liabilities

FAIR VALUE OF FINANCIAL ASSETS AND LIABILITIES

The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicate the level of the fair value hierarchy used to determine such fair values:


	Fair Value Measurements as of March 31, 2023 Using:
(in thousands)	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	70,959	$	14	$	—	$	70,973
	$	70,959	$	14	$	—	$	70,973
Liabilities:
Embedded derivative liability	$	—	$	—	$	10	$	10
Class C warrant liability	—		—		17,692		17,692
	$	—	$	—	$	17,702	$	17,702


	Fair Value Measurements as of December 31, 2022 Using:
(in thousands)	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	70,170	$	2,858	$	—	$	73,028
	$	70,170	$	2,858	$	—	$	73,028
Liabilities:
Embedded derivative liability	$	—	$	—	$	10	$	10
Class C warrant liability	—		—		23,131		23,131
	$	—	$	—	$	23,141	$	23,141

The Company’s cash equivalents consisted of money market funds invested primarily in short term commercial paper, asset- backed securities, certificate of deposits and repurchase agreements. The money market funds were valued based on reported market pricing for the identical assets, which represents a Level 1 measurement, or by using inputs observable in active markets for similar securities, which represents a Level 2 measurement.

The following table provides a roll-forward of the aggregate fair values financial instruments for which fair values are determined using Level 3 inputs:


(in thousands)	Embedded Derivative Liability		Class C Warrant Liability		Total
Balance as of December 31, 2022	$	10	$	23,131	$	23,141

Change in fair value	—		(5,439)		(5,439)

Balance as of March 31, 2023	$	10	$	17,692	$	17,702

Embedded Derivative Liability— The fair value of the embedded derivative liability recognized in connection with the Company’s loan agreement with Hercules (see Note 7), which is associated with additional fees due to Hercules upon events of default, was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of this embedded derivative liability, which is reported within other non-current liabilities on the condensed consolidated balance sheets, is estimated by the Company at each reporting date based, in part, on the results of third party valuations, which are prepared based on a discounted cash flow model that considers the timing and probability of occurrence of a redemption upon an event of default, the potential amount of prepayment fees or contingent interest upon an event of default and the Company’s risk-adjusted discount rate of 14%.

Class C Warrant Liability— In December 2022, the Company issued Class C Warrants for the purchase of shares of its common stock in a public offering of common stock. The Class C Warrants are accounted for as a liability on the condensed consolidated balance sheet and are adjusted to fair value at period end through “other income (expense)” in the condensed, consolidated statements of operations and comprehensive loss.

The Company calculated the fair value of the Class C Warrants using the Black-Scholes option pricing model, which represents a Level 3 measurement within the fair value hierarchy, with the following inputs:


	March 31, 2023		December 31, 2022
Common stock price	$0.87		$0.99
Risk-free interest rate	3.9	%	4.0	%
Expected term (in years)	4.7 years		4.9 years
Expected volatility	91.9	%	101.7	%
Expected dividend yield	—	%	—	%

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Property and Equipment, Net

3 Months Ended

Mar. 31, 2023

Property, Plant and Equipment [Abstract]

Property and Equipment, Net

PROPERTY AND EQUIPMENT, NET

Property and equipment, net consisted of the following:


(in thousands)	March 31, 2023		December 31, 2022
Leasehold improvements	$	228	$	228
Furniture and fixtures	1,274		1,268
Computer equipment	173		173
Software	24		24
Lab equipment	642		639
	2,341		2,332
Less: Accumulated depreciation and amortization	(1,355)		(1,228)
	$	986	$	1,104

Depreciation and amortization expense related to property and equipment was $127 thousand and $133 thousand for the three months ended March 31, 2023 and 2022, respectively.

Accrued Expenses

3 Months Ended

Mar. 31, 2023

Payables and Accruals [Abstract]

Accrued Expenses

ACCRUED EXPENSESAccrued expenses consisted of the following:


(in thousands)	March 31, 2023		December 31, 2022
Accrued employee compensation and benefits	$	2,742	6,592
Accrued external research and development expenses	7,721		3,906
Accrued professional fees	924		571
Accrued deferred financing fees	—		591
Other	375		374
	$	11,762	$	12,034

Long-Term Debt

3 Months Ended

Mar. 31, 2023

Debt Disclosure [Abstract]

Long-Term Debt

Long-term debt consisted of the following:


(in thousands)	March 31, 2023		December 31, 2022
Principal amount of long-term debt	$	32,500	$	32,500
Debt discount, net of accretion	(767)		(196)
Cumulative accretion of end of term payments	648		1,315
Long-term debt	$	32,381	$	33,619
Less: current portion	$	(554)	$	(1,315)
Long-term debt, net of current portion	$	31,827	$	32,304

Hercules Loan Agreement, As Amended

In October 2018, the Company entered into a Loan and Security Agreement (the “Hercules Loan Agreement”), as amended most recently in January 2023, with Hercules, under which the Company has borrowed an aggregate of $32.5 million of term loans to date representing the maximum borrowings. Borrowings under the Hercules Loan Agreement accrues interest at a variable rate equal to the greater of (i) 10.15% or (ii) The Wall Street Journal prime rate plus 3.15%. In an event of default and until such event is no longer continuing, the interest rate applicable to borrowings would be increased by 4.0%.

Borrowings under the Hercules Loan Agreement are repayable in monthly interest-only payments through September 2024, and in equal monthly payments of principal and accrued interest from October 1, 2024 until the maturity date of the loan on April 1, 2026; provided, however, if certain conditions are met and the Company achieves certain operational and financial milestones,

the maturity date will be extended to July 1, 2027. The Company may prepay all, but not less than all, of the outstanding borrowings, subject to a prepayment premium of 3.0% of the principal amount outstanding as of the date of repayment. In addition, the Hercules Loan Agreement provides for payments of $0.8 million and $1.3 million payable on July 1, 2023 and April 1, 2026, respectively, which payments are accelerated upon the prepayment of the borrowings upon the Company’s election or upon default of the loan. Borrowings under the Hercules Loan Agreement are collateralized by substantially all of the Company’s personal property and other assets except for its intellectual property (but including rights to payment and proceeds from the sale, licensing or disposition of the intellectual property).

Under the Hercules Loan Agreement, the Company has agreed to affirmative and negative covenants. Pursuant to the Hercules Loan Agreement, the Company is required to maintain cash in an account or accounts in which Hercules has a first priority security interest, in an aggregate amount greater than $20.0 million. Upon the FDA’s approval of the sale and marketing of mavorixafor for the treatment to patients with WHIM syndrome with a label claim that is generally consistent with that sought in the Company’s New Drug Application filing, the required level shall be reduced to $10.0 million. A breach of any of the covenants under the Hercules Loan Agreement could result in a default under the loan. Upon the occurrence of an event of default under the loan facility with Hercules, the lender could elect to declare all amounts outstanding, if any, to be immediately due and payable and terminate all commitments to extend further credit. If there are any amounts outstanding that the Company is unable to repay, Hercules could proceed against the collateral to secure such indebtedness.

The Hercules Loan Agreement also restricts the Company’s ability to incur additional indebtedness, pay dividends, encumber its intellectual property, or engage in certain fundamental business transactions, such as mergers or acquisitions of other businesses, with certain exceptions.

The Company recognized interest expense under the Hercules Loan Agreement as follows:


(in thousands)	Three Months Ended March 31,
	2023		2022
Total interest expense	$	884	$	909
Non-cash interest expense	$	225	$	198

The annual effective interest rate of the Hercules Loan Agreement as of March 31, 2023 is 13.8%. There were no principal payments due or paid under the Hercules Loan Agreement during the three months ended March 31, 2023. An end-of-term payment of $1.3 million was paid during the three months ended March 31, 2023 in conjunction with the second amendment and restatement of the Hercules Loan Agreement, which was entered into on January 6, 2023. The Company concluded that the amendment and restatement represented a modification to the debt. Accordingly, fees paid to third parties directly related to the amended and restated debt were expensed as incurred and fees paid to Hercules in conjunction with the amendment and restatement were deferred and are being amortized to interest expense over the life of the debt arrangement using the effective interest method.

As of March 31, 2023, future principal and accrued end-of-term payments due under the Hercules Loan Agreement were as follows (in thousands):


Year Ending December 31,	Total
2023	$	554
2024	4,750
2025	20,386
2026	7,458
Long-term debt	$	33,148

Leases

3 Months Ended

Mar. 31, 2023

Leases [Abstract]

Leases

LEASES

The Company has lease agreements for its facilities in Boston, Massachusetts, which is the Company’s principal executive office; Vienna, Austria, which is the Company’s research and development center; and Waltham, Massachusetts, which the Company has sublet to a third party. There are no restrictions or financial covenants associated with any of the lease agreements.

•Vienna Austria Leases— The Company has an operating lease for approximately 1,200 square meters of laboratory and office space in Vienna, Austria (“Vienna Lease”), which commenced in February 2021 for a term of 7 years. The annual base rent for the Vienna Lease is approximately $285 thousand.

•Boston Lease— The Company leases approximately 28,000 square feet of office space in Boston, Massachusetts (“Boston Lease”), which serves as the Company’s headquarters. Base rental payments are approximately $1.1 million annually, plus certain operating expenses. The term of the Boston Lease will continue until November 2026, unless earlier terminated. The Company has the right to sublease the premises, subject to landlord consent and also has the right to renew the Boston Lease for an additional five years at the then prevailing effective market rental rate. The Company is required to maintain a security deposit in the form of a letter of credit for $0.6 million for the benefit of the landlord.

•Waltham Lease— The Company leases approximately 6,000 square feet of office space in Waltham, Massachusetts (“Waltham Lease”). The Waltham Lease, as amended, commenced on January 1, 2019, and expires on December 31, 2023. The base rent is approximately $0.3 million annually. In addition to the base rent, the Company is also responsible for its share of operating expenses, electricity and real estate taxes, which costs are not included in the determination of the leases’ right-of-use assets or lease liabilities. The Company is subleasing the space to a third party for the duration of the lease. The right-of-use asset is being amortized to rent expense over the five-year term of the lease.

As the Company’s leases do not provide an implicit rate, the Company estimated the incremental borrowing rate in calculating the present value of the lease payments. The Company utilizes its incremental borrowing rates, which are the rates incurred to borrow on a collateralized basis over a similar term and amount equal to the lease payments in a similar economic environment.

The components of lease expense for the three months ended March 31, 2023 and 2022 were as follows:


(dollars in thousands)	Three Months Ended March 31,
Lease Cost	2023			2022
Fixed operating lease cost	$	522		$	530

Total lease expense	$	522		$	530
Other information

Operating cash flows from operating leases	$	346		$	342
Sublease income	$	49		$	49
Weighted-average remaining lease term—operating leases	3.8 years
Weighted-average discount rate—operating leases	11.3		%

Maturities of lease liabilities due under lease agreements that have commenced as of March 31, 2023 are as follows (in thousands)


Maturity of lease liabilities	Operating Leases
2023 (remainder of the year)	$	1,210
2024	1,378
2025	1,406
2026	1,337
2027	285
Thereafter	48
Total lease payments	5,664
Less: interest	(1,112)
Total operating lease liabilities as of March 31, 2023	$	4,552

Commitment and Contingencies	3 Months Ended
Commitment and Contingencies	Mar. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitment and Contingencies	COMMITMENTS AND CONTINGENCIES The Company has agreements with contract manufacturing organizations (“CMOs”) for the production of mavorixafor for use in clinical trials. The Company’s agreement with the CMO who produces batches of drug substance for use in the Company’s clinical drug supply contains cancellation provisions that would require the Company to pay up to the full contract value upon cancellation. As of March 31, 2023, the Company has approximately $0.8 million of such commitments in place subject to cancellation provisions. Indemnification Agreements— In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its executive officers that will require the Company to, among other things, indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnification obligations. The Company is not currently aware of any indemnification claims and has not accrued any liabilities related to such obligations in its condensed consolidated financial statements as of March 31, 2023 or December 31, 2022. Legal Proceedings— The Company is not a party to any litigation and does not have contingency reserves established for any litigation liabilities. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses as incurred the costs related to any legal proceedings.

Common Stock Warrants

3 Months Ended

Mar. 31, 2023

Equity [Abstract]

Common Stock Warrants

COMMON STOCK AND COMMON STOCK WARRANTS

As of March 31, 2023, the Company’s Restated Certificate of Incorporation authorized the Company to issue 500 million shares of common stock, par value $0.001 per share. The voting, dividend and liquidation rights of the holders of the Company’s common stock are subject to and qualified by the rights, powers and preferences of the holders of any preferred stock that may be issued. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to receive dividends, as may be declared by the board of directors, if any. No cash dividends have been declared or paid to date.

In connection with previous public and private sales of shares of its common stock, the Company has issued warrants and pre-funded warrants, which are exercisable for the purchase shares of the Company’s common stock. All outstanding warrants and pre-funded warrants are currently exercisable and do not have price reset provisions. Upon the closing of these public and

private offerings, the Company received approximately 99% of the exercise price for the pre-funded warrants, for which the remaining exercise price is equal to or less than $0.01 per share.

As of March 31, 2023, the Company’s outstanding warrants and pre-funded warrants to purchase shares of common stock consisted of the following:


Issuance Date	Number of Shares of Common Stock Issuable	Exercise Price			Expiration Date
October 25, 2016	5,155	$	19.78		October 24, 2026
December 28, 2017	115,916	$	19.78		December 28, 2027
September 12, 2018	20,220	$	19.78		September 12, 2028
October 19, 2018	20,016	$	19.78		October 19, 2028
March 13, 2019	5,000	$	19.78		March 12, 2029
April 16, 2019	3,866,154	$	13.20		April 15, 2024

November 29, 2019	1,250,000	$	12.00	(a)	n/a
March 23, 2021	50,000	$	8.70	(b)	n/a
November 9, 2021	2,008,032	$	4.98	(c)	n/a
March 3, 2022	766,666	$	1.80	(d)	n/a
July 6, 2022	13,276,279	$	1.095	(e)	n/a
July 6, 2022	50,925,365	$	1.095		July 6, 2027
December 9, 2022	32,762,947	$	1.50		December 9, 2027
December 9, 2022	6,800,000	$	1.10	(f)	n/a
	111,871,750

(a) In November 2019, the Company received $11.999 per pre-funded warrant, or $21.0 million in aggregate proceeds. Each pre-funded

warrant may be exercised for an additional $0.001 per pre-funded warrant. (b) In March 2021, the Company received $8.69 per pre-funded warrant, or $435 thousand in aggregate proceeds. Each pre-funded warrant may be exercised for an additional $0.01 per pre-funded warrant. (c) In November 2021, the Company received $4.97 per pre-funded warrant, or $10.0 million in aggregate proceeds. Each pre-funded warrant may be exercised for an additional $0.01 per pre-funded warrant. (d) In March 2022, the Company received $1.79 per pre-funded warrant, or $1.4 million in aggregate proceeds. Each pre-funded warrant may be exercised for an additional $0.01 per pre-funded warrant. (e) In July 2022, the Company received $1.094 per pre-funded warrant, or $14.5 million in aggregate proceeds. Each pre-funded warrant may be exercised for an additional $0.001 per pre-funded warrant. (f) In December 2022, the Company received $1.099 per pre-funded warrant, or $7.5 million in aggregate proceeds. Each pre-funded warrant may be exercised for an additional $0.001 per pre-funded warrant.

Stock-Based Compensation

3 Months Ended

Mar. 31, 2023

Share-Based Payment Arrangement [Abstract]

Stock-Based Compensation

STOCK-BASED COMPENSATION

Summary of Plans— The Company has the following equity incentive plans:

•The X4 Pharmaceuticals Inc. 2015 Employee, Director and Consultant Equity Incentive Plan, as amended (the “2015 Plan”);

•The X4 Pharmaceuticals Inc. 2017 Equity Incentive Plan (the “2017 Plan”); and

•The X4 Pharmaceuticals Inc. 2019 Inducement Equity Incentive Plan (the “2019 Plan”)

The Company also has the following employee stock purchase plan:

•The X4 Pharmaceutical Inc. 2017 Employee Stock Purchase Plan (the “2017 ESPP”)

These plans are administered by the Board of Directors or by a committee of the Board of Directors. The exercise prices, vesting and other restrictions are determined at the discretion of the Board of Directors, or its committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant and the term of the stock option may not be greater than ten years. Incentive stock options granted to employees and restricted stock awards granted to employees, officers, members of the board of directors, advisors, and consultants of the Company typically vest over four years. Non-statutory options granted to employees, officers, members of the board of directors, advisors, and consultants of the Company typically vest over three or four years. Shares that are expired, terminated, surrendered or canceled under the Plans without having been fully exercised will be available for future awards. In

addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for the grant of awards.

As of March 31, 2023, there are an aggregate of approximately 1.0 million shares of common stock available for issuance under the Company’s equity incentive plans. Approximately 150,000 shares of common stock remain available for issuance under the 2017 ESPP.

Stock Option Valuation— The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted to employees, directors and non-employees.


	Three Months Ended March 31,
	2023		2022
Risk-free interest rate	3.6	%	2.0	%
Expected term (in years)	6.0		6.1
Expected volatility	90.8	%	94.5	%
Expected dividend yield	0	%	0	%

Stock Options

The following table summarizes the Company’s stock option activity for the three months ended March 31, 2023:


	Number of Shares	Weighted Average Exercise Price		Weighted Average Contractual Term (Years)	Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2022	2,021,480	$	6.99	6.5	$	1
Granted	834,900	0.90

Forfeited and Expired	(26,080)	8.68
Outstanding as of March 31, 2023	2,830,300	$	5.18	8.0	$	—
Exercisable as of March 31, 2023	1,163,500	$	9.72	6.1	$	—
Vested and expected to vest as of March 31, 2023	2,210,011	$	6.20	7.6	$	—

There were no stock options exercised in the three months ended March 31, 2023 and 2022. The weighted average grant-date fair value per share of stock options granted during the three months ended March 31, 2023 and 2022 was $0.69 and $1.42, respectively.

Restricted Stock Units— The following table summarizes the Company's restricted stock unit activity for the three months ended March 31, 2023:


			Number of Shares
Unvested as of December 31, 2022			1,680,563
Granted			4,694,691
Vested			(540,238)

Unvested as of March 31, 2023			5,835,016

During the three months ended March 31, 2023, the Company granted performance-based restricted stock units (“PRSUs”) to its employees. The PRSUs vest 50% based on the Company’s achievement of each of two operational milestones conditioned on the grantee’s continued employment with the Company. As of March 31, 2023, neither of the two performance criteria had been met. The Company believes that the achievement of these operational milestones is probable and, accordingly, stock-based compensation expense has been recognized for the awards using the accelerated attribution model based on the fair value of the awards as of the date of grant and management’s best estimate of the date each operational milestone will be achieved. The

Company will update its estimates related to the probability and timing of achievement of the operational milestones each period until the award either vests or is forfeited.

Stock-Based Compensation— As of March 31, 2023, total unrecognized compensation expense related to unvested stock options and restricted stock units was $6.2 million, which is expected to be recognized over a weighted average period of 1.3 years.

Stock-based compensation expense was classified in the condensed consolidated statements of operations as follows:


	Three Months Ended March 31,
(in thousands)	2023		2022
Research and development expense	$	831	$	704
Selling, general and administrative expense	814		755
Total stock-based compensation	$	1,645	$	1,459

Income Taxes	3 Months Ended
Income Taxes	Mar. 31, 2023
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXESThe Company did not record a U.S. federal or state income tax benefit for its losses for the three months ended March 31, 2023 and 2022, due to the conclusion that a full valuation allowance is required against the Company’s U.S. federal and state deferred tax assets. For the three months ended March 31, 2023 and 2022, the Company recorded an immaterial income tax provision related to its Austrian subsidiary.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 21
-URI https://asc.fasb.org/extlink&oid=121826272&loc=d3e32857-109319

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 11.C)
-URI https://asc.fasb.org/extlink&oid=122134291&loc=d3e330215-122817

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 17
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=121826272&loc=d3e32809-109319

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 6.I.5.Q1)
-URI https://asc.fasb.org/extlink&oid=122134291&loc=d3e330036-122817

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 14
-URI https://asc.fasb.org/extlink&oid=121826272&loc=d3e32705-109319

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 270
-Section 50
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=6424409&loc=d3e44925-109338

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-URI https://asc.fasb.org/topic&trid=2144680

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=6424122&loc=d3e41874-109331

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(h)(2))
-URI https://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

+ Details

Net Loss per Share

3 Months Ended

Mar. 31, 2023

Earnings Per Share [Abstract]

Net Loss per Share

NET LOSS PER SHARE

Basic and diluted net loss per share attributable to common stockholders was calculated as follow:


	Three Months Ended March 31,
(in thousands, except per share data)	2023		2022
Numerator:
Net loss	$	(24,020)	$	(21,965)
Deemed dividend as a result of Class B warrant price reset	—		(2,259)
Net loss attributable to common stockholders	$	(24,020)	$	(24,224)
Denominator:
Weighted average shares of common stock outstanding—basic and diluted	145,967		33,737
Net loss per share attributable to common stockholders— basic and diluted	$	(0.16)	$	(0.72)

Basic and diluted weighted average shares of common stock outstanding for the three months ended March 31, 2023 and March 31, 2022 include the weighted average effect of outstanding pre-funded warrants for the purchase of shares of common stock for which the remaining unfunded exercise price is $0.01 or less per share. During the three months ended March 31, 2022, in accordance with the Company’s Class B Warrant agreement, the exercise price of each outstanding Class B Warrant was adjusted to the price of shares of the Company’s common stock sold in public or private offerings to the extent such price is lower than the previous Class B warrant price. These price adjustments were accounted for as a deemed dividend that adjusts net loss available to common shareholders for purposes of basic earnings per share. The deemed dividend was calculated using the Black-Scholes pricing model, taking into account historical volatility of the Company’s common stock and the estimated remaining life of the outstanding Class B Warrants. The Class B Warrants expired in December 2022.

The Company’s potentially dilutive securities include outstanding stock options, unvested restricted stock units and warrants to purchase shares of common stock for the three months ended March 31, 2023 and 2022. All potentially dilutive securities have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share, and thus they are considered “anti-dilutive.” Therefore, the weighted average number of shares of common stock outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following potential shares of common stock, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:


	Three Months Ended March 31,
	2023	2022
Options to purchase shares of common stock	2,830,300	2,049,158
Unvested restricted stock units	5,835,016	1,940,788
Warrants to purchase shares of common stock (excluding prefunded warrants, which are included in basic shares outstanding)	87,720,773	9,449,028
	96,386,089	13,438,974

Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2023
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation—The condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, including X4 Pharmaceuticals (Austria) GmbH, which is incorporated in Vienna, Austria (“X4 Austria”), and X4 Therapeutics, Inc. All significant intercompany accounts and transactions have been eliminated.
Use of Estimates	Use of Estimates— The preparation of the Company’s consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses, the impairment or lack of impairment of long-lived assets including operating lease right-of-use assets and goodwill, and assumptions underlying the fair value of warrant liabilities. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates when there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. As of the date of issuance of these consolidated financial statements, the Company is not aware of any specific event or circumstance that would require the Company to update its estimates, assumptions and judgments or revise the carrying value of its assets or liabilities. Actual results could differ from those estimates, and any such differences may be material to the Company’s consolidated financial statements.
Cash and Cash Equivalents	Cash and Cash Equivalents— The Company considers all highly liquid investments with maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents consisted of money market funds as of March 31, 2023 and December 31, 2022.
Goodwill	Goodwill— Goodwill is tested for impairment at the reporting unit level annually in the fourth quarter, or more frequently when events or changes in circumstances indicate that the asset might be impaired. Examples of such events or circumstances include, but are not limited to, a significant adverse change in legal or business climate, an adverse regulatory action or unanticipated competition. The Company has determined that it operates in a single operating segment and has a single reporting unit. The Company assesses qualitative factors to determine whether the existence of events or circumstances would indicate that it is more likely than not that the fair value of the reporting unit is less than its carrying amount. If after assessing the totality of events or circumstances, the Company were to determine that it is more likely than not that the fair value of the reporting unit is less than its carrying amount, then the Company would perform an interim quantitative impairment test, whereby the Company compares the fair value of the reporting unit to its carrying value. If the fair value of the reporting unit exceeds the carrying value of its net assets, goodwill is not impaired, and no further testing is required. If the fair value of the reporting unit is less than its carrying value, the Company measures the amount of impairment loss, if any, as the excess of the carrying value over the fair value of the reporting unit. There were no triggering events during the three months ended March 31, 2023 that necessitated an interim impairment test of goodwill.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting StandardsIn June 2016, the Financial Accounting Standards Board issued ASU 2016-13, Credit Losses (Topic 326) Measurement of Credit Losses on Financial Instruments (“ASU 2016-13”), as amended. ASU 2016-13 requires that financial assets measured at amortized cost, such as trade receivables, be presented net of expected credit losses, which may be estimated based on relevant information such as historical experience, current conditions, and future expectation for each pool of similar financial asset. The new guidance requires enhanced disclosures related to trade receivables and associated credit losses. The Company adopted ASU 2016-13 on January 1, 2023. As the Company did not have accounts receivable on its consolidated balance sheet as of the date of adoption, there was no impact to the adoption of ASU 2016-13.
Research and Development Expense, Policy

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2023

Accounting Policies [Abstract]

Schedule of Restricted Cash

Restricted Cash


(in thousands)	As of March 31, 2023		As of December 31, 2022

Letter of credit security: Waltham lease	$	250	$	250
Letter of credit security: Vienna Austria lease	206		205
Letter of credit security: Boston lease	571		855
Total restricted cash	$	1,027	$	1,310
Restricted cash included in prepaid expenses and other current assets	$	250	$	285
Restricted cash included in other assets	$	777	$	1,025

Schedule of Reconciliation of Cash, Cash Equivalents, and Restricted Cash


(in thousands)	March 31, 2023		December 31, 2022
Cash and cash equivalents	$	93,406	$	121,718
Restricted cash, current portion	250		285
Restricted cash, non-current	777		1,025
Total cash, cash equivalents and restricted cash	$	94,433	$	123,028

Fair Value of Financial Assets and Liabilities (Tables)

3 Months Ended

Mar. 31, 2023

Fair Value Disclosures [Abstract]

Schedule of Assets and Liabilities Measured at Fair Value


	Fair Value Measurements as of March 31, 2023 Using:
(in thousands)	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	70,959	$	14	$	—	$	70,973
	$	70,959	$	14	$	—	$	70,973
Liabilities:
Embedded derivative liability	$	—	$	—	$	10	$	10
Class C warrant liability	—		—		17,692		17,692
	$	—	$	—	$	17,702	$	17,702


	Fair Value Measurements as of December 31, 2022 Using:
(in thousands)	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	70,170	$	2,858	$	—	$	73,028
	$	70,170	$	2,858	$	—	$	73,028
Liabilities:
Embedded derivative liability	$	—	$	—	$	10	$	10
Class C warrant liability	—		—		23,131		23,131
	$	—	$	—	$	23,141	$	23,141

Summary of Aggregate Fair Values of Warrant Liability and Derivative Liability

The following table provides a roll-forward of the aggregate fair values financial instruments for which fair values are determined using Level 3 inputs:


(in thousands)	Embedded Derivative Liability		Class C Warrant Liability		Total
Balance as of December 31, 2022	$	10	$	23,131	$	23,141

Change in fair value	—		(5,439)		(5,439)

Balance as of March 31, 2023	$	10	$	17,692	$	17,702

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions


	March 31, 2023		December 31, 2022
Common stock price	$0.87		$0.99
Risk-free interest rate	3.9	%	4.0	%
Expected term (in years)	4.7 years		4.9 years
Expected volatility	91.9	%	101.7	%
Expected dividend yield	—	%	—	%

Property and Equipment, Net (Tables)

3 Months Ended

Mar. 31, 2023

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment, net consisted of the following:


(in thousands)	March 31, 2023		December 31, 2022
Leasehold improvements	$	228	$	228
Furniture and fixtures	1,274		1,268
Computer equipment	173		173
Software	24		24
Lab equipment	642		639
	2,341		2,332
Less: Accumulated depreciation and amortization	(1,355)		(1,228)
	$	986	$	1,104

Accrued Expenses (Tables)

3 Months Ended

Mar. 31, 2023

Payables and Accruals [Abstract]

Schedule of Accrued Expenses

Accrued expenses consisted of the following:


(in thousands)	March 31, 2023		December 31, 2022
Accrued employee compensation and benefits	$	2,742	6,592
Accrued external research and development expenses	7,721		3,906
Accrued professional fees	924		571
Accrued deferred financing fees	—		591
Other	375		374
	$	11,762	$	12,034

Long-Term Debt (Tables)

3 Months Ended

Mar. 31, 2023

Debt Disclosure [Abstract]

Summary of Long Term Debt

Long-term debt consisted of the following:


(in thousands)	March 31, 2023		December 31, 2022
Principal amount of long-term debt	$	32,500	$	32,500
Debt discount, net of accretion	(767)		(196)
Cumulative accretion of end of term payments	648		1,315
Long-term debt	$	32,381	$	33,619
Less: current portion	$	(554)	$	(1,315)
Long-term debt, net of current portion	$	31,827	$	32,304

Interest Income and Interest Expense Disclosure

The Company recognized interest expense under the Hercules Loan Agreement as follows:


(in thousands)	Three Months Ended March 31,
	2023		2022
Total interest expense	$	884	$	909
Non-cash interest expense	$	225	$	198

Schedule of Future Principal Payments and the Final Payments Due

As of March 31, 2023, future principal and accrued end-of-term payments due under the Hercules Loan Agreement were as follows (in thousands):


Year Ending December 31,	Total
2023	$	554
2024	4,750
2025	20,386
2026	7,458
Long-term debt	$	33,148

Leases (Tables)

3 Months Ended

Mar. 31, 2023

Leases [Abstract]

Schedule of Components of Lease Expense

The components of lease expense for the three months ended March 31, 2023 and 2022 were as follows:


(dollars in thousands)	Three Months Ended March 31,
Lease Cost	2023			2022
Fixed operating lease cost	$	522		$	530

Total lease expense	$	522		$	530
Other information

Operating cash flows from operating leases	$	346		$	342
Sublease income	$	49		$	49
Weighted-average remaining lease term—operating leases	3.8 years
Weighted-average discount rate—operating leases	11.3		%

Schedule of Maturities of Lease Liabilities

Maturities of lease liabilities due under lease agreements that have commenced as of March 31, 2023 are as follows (in thousands)


Maturity of lease liabilities	Operating Leases
2023 (remainder of the year)	$	1,210
2024	1,378
2025	1,406
2026	1,337
2027	285
Thereafter	48
Total lease payments	5,664
Less: interest	(1,112)
Total operating lease liabilities as of March 31, 2023	$	4,552

Common Stock Warrants (Tables)

3 Months Ended

Mar. 31, 2023

Equity [Abstract]

Schedule of Warrants

As of March 31, 2023, the Company’s outstanding warrants and pre-funded warrants to purchase shares of common stock consisted of the following:


Issuance Date	Number of Shares of Common Stock Issuable	Exercise Price			Expiration Date
October 25, 2016	5,155	$	19.78		October 24, 2026
December 28, 2017	115,916	$	19.78		December 28, 2027
September 12, 2018	20,220	$	19.78		September 12, 2028
October 19, 2018	20,016	$	19.78		October 19, 2028
March 13, 2019	5,000	$	19.78		March 12, 2029
April 16, 2019	3,866,154	$	13.20		April 15, 2024

November 29, 2019	1,250,000	$	12.00	(a)	n/a
March 23, 2021	50,000	$	8.70	(b)	n/a
November 9, 2021	2,008,032	$	4.98	(c)	n/a
March 3, 2022	766,666	$	1.80	(d)	n/a
July 6, 2022	13,276,279	$	1.095	(e)	n/a
July 6, 2022	50,925,365	$	1.095		July 6, 2027
December 9, 2022	32,762,947	$	1.50		December 9, 2027
December 9, 2022	6,800,000	$	1.10	(f)	n/a
	111,871,750

(a) In November 2019, the Company received $11.999 per pre-funded warrant, or $21.0 million in aggregate proceeds. Each pre-funded

Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2023

Share-Based Payment Arrangement [Abstract]

Stock Option Valuation

The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted to employees, directors and non-employees.


	Three Months Ended March 31,
	2023		2022
Risk-free interest rate	3.6	%	2.0	%
Expected term (in years)	6.0		6.1
Expected volatility	90.8	%	94.5	%
Expected dividend yield	0	%	0	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the three months ended March 31, 2023:


	Number of Shares	Weighted Average Exercise Price		Weighted Average Contractual Term (Years)	Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2022	2,021,480	$	6.99	6.5	$	1
Granted	834,900	0.90

Forfeited and Expired	(26,080)	8.68
Outstanding as of March 31, 2023	2,830,300	$	5.18	8.0	$	—
Exercisable as of March 31, 2023	1,163,500	$	9.72	6.1	$	—
Vested and expected to vest as of March 31, 2023	2,210,011	$	6.20	7.6	$	—

Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity

Restricted Stock Units— The following table summarizes the Company's restricted stock unit activity for the three months ended March 31, 2023:


			Number of Shares
Unvested as of December 31, 2022			1,680,563
Granted			4,694,691
Vested			(540,238)

Unvested as of March 31, 2023			5,835,016

Summary of Stock-Based Compensation Expense Classification

Stock-based compensation expense was classified in the condensed consolidated statements of operations as follows:


	Three Months Ended March 31,
(in thousands)	2023		2022
Research and development expense	$	831	$	704
Selling, general and administrative expense	814		755
Total stock-based compensation	$	1,645	$	1,459

Share-Based Payment Arrangement, Option and Stock Appreciation Rights, Activity

The calculation of the fair value of the outstanding SARs as of March 31, 2023 includes the closing price of the Company’s common stock of $0.87 and the following assumptions on a weighted average basis:


	March 31, 2023
Risk free rate	3.6	%
Expected term (years)	5.61
Expected volatility	90.8	%
Expected dividend yield	—	%
Expected forfeiture rate	22	%

Net Loss per Share (Tables)

3 Months Ended

Mar. 31, 2023

Earnings Per Share [Abstract]

Summary of Basic and Diluted Net Loss per Share Attributable to Common Stockholders

Basic and diluted net loss per share attributable to common stockholders was calculated as follow:


	Three Months Ended March 31,
(in thousands, except per share data)	2023		2022
Numerator:
Net loss	$	(24,020)	$	(21,965)
Deemed dividend as a result of Class B warrant price reset	—		(2,259)
Net loss attributable to common stockholders	$	(24,020)	$	(24,224)
Denominator:
Weighted average shares of common stock outstanding—basic and diluted	145,967		33,737
Net loss per share attributable to common stockholders— basic and diluted	$	(0.16)	$	(0.72)

Schedule of Anti-dilutive Securities Excluded from Computation of Diluted Net Loss per Share Attributable to Common Stockholders

The Company excluded the following potential shares of common stock, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:


	Three Months Ended March 31,
	2023	2022
Options to purchase shares of common stock	2,830,300	2,049,158
Unvested restricted stock units	5,835,016	1,940,788
Warrants to purchase shares of common stock (excluding prefunded warrants, which are included in basic shares outstanding)	87,720,773	9,449,028
	96,386,089	13,438,974

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022
Business Acquisition [Line Items]
Cash and cash equivalents	$ 93,406		$ 121,718
Accumulated deficit	(400,758)		$ (376,738)
Net cash used in operating activities	$ (26,512)	$ (20,229)

Summary of Significant Accounting Policies - Schedule of Compensating Balance Arrangements (Detail) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Cash and Cash Equivalents [Line Items]
Total restricted cash	$ 777	$ 1,025
Restricted Cash	1,027	1,310
Restricted cash, current portion	250	285
Letter of Credit \| Waltham Lease
Cash and Cash Equivalents [Line Items]
Restricted Cash	250	250
Letter of Credit \| Vienna Austria Lease
Cash and Cash Equivalents [Line Items]
Restricted Cash	206	205
Letter of Credit \| Allston Lease Agreement
Cash and Cash Equivalents [Line Items]
Restricted Cash	$ 571	$ 855

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Accounting Policies [Abstract]
Cash and cash equivalents	$ 93,406	$ 121,718
Restricted cash, current portion	250	285
Restricted cash, non-current	777	1,025
Total cash, cash equivalents and restricted cash	$ 94,433	$ 123,028	$ 67,744	$ 83,108

License, Collaboration, and Funding Agreements - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Collaboration License And Funding Arrangements [Line Items]
Other income, net	$ 123,000	$ 60,000
Gain (Loss) on Disposition of Other Assets	500,000
Research and Development Incentive
Collaboration License And Funding Arrangements [Line Items]
Grant receivable	700,000
Other income, net	$ 121,000	$ 135,300

Fair Value of Financial Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	$ 70,973	$ 73,028
Fair value of derivative liability	17,702	23,141
Embedded derivative liability
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value of derivative liability	10	10
Class C Warrant Liability
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value of derivative liability	17,692	23,131
Level 1
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	70,959	70,170
Fair value of derivative liability	0	0
Level 1 \| Embedded derivative liability
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value of derivative liability	0	0
Level 2
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	14	2,858
Fair value of derivative liability	0	0
Level 2 \| Embedded derivative liability
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value of derivative liability	0	0
Level 3
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	0	0
Fair value of derivative liability	17,702	23,141
Level 3 \| Embedded derivative liability
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value of derivative liability	10	10
Level 3 \| Class C Warrant Liability
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value of derivative liability	$ 17,692	$ 23,131

Fair Value of Financial Assets and Liabilities - Aggregate Fair Values Financial Instruments (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2023 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 23,141
Change in fair value	(5,439)
Ending balance	17,702
Embedded Derivative Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	10
Change in fair value	0
Ending balance	10
Class C Warrant Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	23,131
Change in fair value	(5,439)
Ending balance	$ 17,692

Fair Value of Financial Assets and Liabilities - Narrative (Details)	Mar. 31, 2023 USD ($)
Measurement Input, Discount Rate
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value of derivative liability	$ 0.14

Fair Value of Financial Assets and Liabilities - Fair Value Assumptions (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended
	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate (in percentage)			3.60%	2.00%
Expected term (in years)			6 years	6 years 1 month 6 days
Expected volatility (in percentage)			90.80%	94.50%
Expected dividend yield (in percentage)			0.00%	0.00%
Fair value of warrant liability	$ 17,702	$ 23,141	$ 17,702
Class C Warrant Liability
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Price (in dollars per share)	$ 0.87	$ 0.99	$ 0.87
Risk-free interest rate (in percentage)	3.90%	4.00%	3.60%
Expected term (in years)	4 years 8 months 12 days	4 years 10 months 24 days	5 years 7 months 9 days
Expected volatility (in percentage)	91.90%	101.70%	90.80%
Expected dividend yield (in percentage)	0.00%	0.00%	0.00%

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 2,341	$ 2,332
Less: Accumulated depreciation and amortization	(1,355)	(1,228)
Property and equipment, net	986	1,104
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	228	228
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,274	1,268
Computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	173	173
Software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	24	24
Lab equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 642	$ 639

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Property, Plant and Equipment [Abstract]
Depreciation and amortization expense	$ 127	$ 133

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Payables and Accruals [Abstract]
Accrued employee compensation and benefits	$ 2,742	$ 6,592
Accrued external research and development expenses	7,721	3,906
Accrued professional fees	924	571
Accrued deferred financing fees	0	591
Other	375	374
Total accrued expenses	$ 11,762	$ 12,034

Long-Term Debt - Summary of Long Term Debt (Detail) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Debt Disclosure [Abstract]
Principal amount of long-term debt	$ 32,500	$ 32,500
Debt discount, net of accretion	(767)	(196)
Cumulative accretion of end of term payments	648	1,315
Long-term debt	32,381	33,619
Current portion of long-term debt	(554)	(1,315)
Long-term debt, net of current portion	$ 31,827	$ 32,304

- Definition

Amount, excluding unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

+ References + Details

Long Term Debt - Hercules Loan Agreement - Additional Information (Detail) - USD ($)			3 Months Ended	30 Months Ended
	Jul. 01, 2024	Jul. 01, 2022	Mar. 31, 2023	Mar. 31, 2021
Debt Instrument [Line Items]
Principal payments			$ 1,300,000
Hercules Loan Agreement
Debt Instrument [Line Items]
Proceeds from lines of credit				$ 32,500,000
Debt instrument variable percentage			10.15%
Interest rate increase percentage			4.00%
Line of credit facility periodic payment		$ 800,000
Effective interest rate of loan			13.80%
Principal payments			$ 0
Hercules Loan Agreement \| Subsequent Event
Debt Instrument [Line Items]
Line of credit facility periodic payment	$ 1,300,000
Hercules Loan Agreement \| Maximum \| Period Two
Debt Instrument [Line Items]
Debt instrument prepayment premium, percentage			3.00%
Hercules Loan Agreement \| Prime Rate
Debt Instrument [Line Items]
Debt instrument variable percentage			3.15%
Hercules Second Amended Loan Agreement \| Min Cash Test Date 1
Debt Instrument [Line Items]
Line of Credit, Covenant, Minimum Cash			$ 20,000,000
Hercules Second Amended Loan Agreement \| Min Cash Test Date 2
Debt Instrument [Line Items]
Line of Credit, Covenant, Minimum Cash			$ 10,000,000

Long-Term Debt - Summary of Interest Expense (Details) - Hercules Loan Agreement - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Debt Instrument [Line Items]
Interest expense, debt	$ 884	$ 909
Amortization of debt discount	$ 225	$ 198

Long-Term Debt - Schedule of Future Principal Payments and the Final Payments Due (Detail) $ in Thousands	Mar. 31, 2023 USD ($)
Debt Disclosure [Abstract]
2023	$ 554
2024	4,750
2024	20,386
2025	7,458
Principal amount of long-term debt	$ 33,148

Leases - Additional Information (Detail) $ in Thousands		3 Months Ended
Leases - Additional Information (Detail) $ in Thousands	Nov. 11, 2019 USD ($) ft²	Mar. 31, 2023 USD ($) m² ft²	Dec. 31, 2022 USD ($)
Lessee, Lease, Description [Line Items]
Operating lease, liability		$ 4,552
Right-of-use assets		$ 6,844	$ 7,229
Waltham Lease
Lessee, Lease, Description [Line Items]
Current office space under lease agreement \| ft²		6,000
Current base rent		$ 300
Lease, term of contract (years)		5 years
Vienna Lease
Lessee, Lease, Description [Line Items]
Office space \| m²		1,200
Lease not yet commenced, term of contract		7 years
Annual base rent		$ 285
Allston Lease
Lessee, Lease, Description [Line Items]
Current office space under lease agreement \| ft²	28,000
Lease not yet commenced	$ 1,100
Lease, renewal term of contract (years)	5 years

Leases - Schedule of Components of Lease Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Leases [Abstract]
Fixed operating lease cost	$ 522	$ 530
Total lease expense	522	530
Operating cash flows from operating leases	346	342
Sublease income	$ 49	$ 49
Weighted-average remaining lease term-operating leases (in years)	3 years 9 months 18 days
Weighted-average discount rate-operating leases	11.30%

Leases - Schedule of Maturities of Lease Liabilities (Detail) $ in Thousands	Mar. 31, 2023 USD ($)
Leases [Abstract]
2020	$ 1,210
2021	1,378
2022	1,406
2023	1,337
2024	285
2025	48
Total lease payments	5,664
Less: interest	(1,112)
Operating lease, liability	$ 4,552

Commitment and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Indemnification Agreements
Contingencies And Commitments [Line Items]
Accrued liabilities	$ 0.0	$ 0.0

Common Stock Warrants - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2023	Dec. 31, 2022	Jul. 06, 2022	Mar. 03, 2022	Mar. 23, 2021	Nov. 02, 2020	Nov. 29, 2019
Class of Warrant or Right [Line Items]
Common stock, shares authorized	500,000,000	500,000,000
Common stock, par value (in usd per share)	$ 0.001	$ 0.001
Dividends on common stock declared or paid	$ 0
Class of Warrant or Right, Additional Exercise Price of Warrants or Rights			$ 0.001	$ 0.01		$ 0.01	$ 0.001
Q1 2022 Private Placement
Class of Warrant or Right [Line Items]
Class of Warrant or Right, Additional Exercise Price of Warrants or Rights					$ 0.01

Common Stock Warrants - Summary of Outstanding Warrants to Purchase Shares of Common Stock (Detail) - USD ($) $ / shares in Units, $ in Thousands	Mar. 31, 2023	Dec. 09, 2022	Jul. 06, 2022	Mar. 03, 2022	Nov. 09, 2021	Mar. 23, 2021	Nov. 02, 2020	Nov. 29, 2019
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	111,871,750
Class Of Warrant Or Right. Aggregate Proceeds From Warrants Issued		$ 7,500	$ 14,500	$ 1,400	$ 10,000	$ 435		$ 21,000
Class of Warrant or Right, Additional Exercise Price of Warrants or Rights			$ 0.001	$ 0.01			$ 0.01	$ 0.001
Funded
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)			$ 1.094	$ 1.79	$ 4.97	$ 8.69		$ 11.999
Issuance On October 25, 2016
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 19.78
Issuance On October 25, 2016 \| Legacy Warrants
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	5,155
Issuance On December 28, 2017 One
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 19.78
Issuance On December 28, 2017 One \| Legacy Warrants
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	115,916
Issuance On September 12, 2018 One
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 19.78
Issuance On September 12, 2018 One \| Legacy Warrants
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	20,220
Issuance On October 19, 2018
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 19.78
Issuance On October 19, 2018 \| Legacy Warrants
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	20,016
Issuance On March 13, 2019
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 19.78
Issuance On March 13, 2019 \| Legacy Warrants
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	5,000
Issuance On April 16, 2019
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 13.20
Issuance On April 16, 2019 \| Class A Warrant
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	3,866,154
Issuance On November 29, 2019 One
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 12.00
Issuance On November 29, 2019 One \| Pre Funded Warrant
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	1,250,000
Issuance On March 23, 2021
Class of Warrant or Right [Line Items]
Exercise Price (usd per share)	$ 8.70
Issuance On March 23, 2021 \| Pre Funded Warrant
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	50,000
Issuance on November Nine Two Thousand Twenty One
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	2,008,032
Exercise Price (usd per share)	$ 4.98
Issuance on March 3 2022
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	766,666
Exercise Price (usd per share)	$ 1.80
Issuance on July 2022 1
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	13,276,279
Exercise Price (usd per share)	$ 1.095
Issuance on July 6 2022 Two
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	50,925,365
Exercise Price (usd per share)	$ 1.095
Issuance on December 9 2022
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	32,762,947
Exercise Price (usd per share)	$ 1.50
Issuance on December 9 2022 2
Class of Warrant or Right [Line Items]
Number of Shares of Common Stock Issuable (in shares)	6,800,000
Exercise Price (usd per share)	$ 1.10

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted, weighted average grant date fair value (in usd per share)	$ 0.69	$ 1.42
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost of stock based awards	$ 6,200
Unrecognized compensation cost of stock based awards, recognition period	1 year 3 months 18 days
2019 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for issuance	1,000,000
Two Thousand Seventeen Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for issuance	150,000

Stock-Based Compensation - Schedule of Assumptions Used in Black-Scholes Option-Pricing Model to Determine Grant-date Fair Value of Stock Options Granted (Detail)			3 Months Ended
	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate (in percentage)			3.60%	2.00%
Expected term (in years)			6 years	6 years 1 month 6 days
Expected volatility (in percentage)			90.80%	94.50%
Expected dividend yield (in percentage)			0.00%	0.00%
Class C Warrant Liability
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate (in percentage)	3.90%	4.00%	3.60%
Expected term (in years)	4 years 8 months 12 days	4 years 10 months 24 days	5 years 7 months 9 days
Expected volatility (in percentage)	91.90%	101.70%	90.80%
Expected dividend yield (in percentage)	0.00%	0.00%	0.00%
Expected forfeiture rate (in percentage)			22.00%

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
	Mar. 31, 2023	Jun. 30, 2022
Number of Shares
Beginning balance (in shares)	2,021,480
Granted (in shares)	834,900
Forfeited (in shares)	(26,080)
Ending balance (in shares)	2,830,300
Number of shares Options, Exercisable (in shares)	1,163,500
Number of shares Options, Vested and expected to vest (in shares)	2,210,011
Weighted Average Exercise Price
Beginning balance (in usd per share)	$ 6.99
Granted (in usd per share)	0.90
Forfeited (in usd per share)	8.68
Ending balance (in usd per share)	5.18
Weighted average exercise price, Exercisable (in usd per share)	9.72
Weighted average exercise price, Vested and expected to vest (in usd per share)	$ 6.20
Weighted average contractual term outstanding (in years)	8 years	6 years 6 months
Weighted average contractual term outstanding, Exercisable (in years)	6 years 1 month 6 days
Weighted average contractual term outstanding, Vested and expected to vest (in years)	7 years 7 months 6 days
Aggregate intrinsic value, Beginning balance	$ 1
Aggregate intrinsic value, Ending balance	0
Aggregate intrinsic value, exercisable	0
Aggregate intrinsic value, vested and expected to vest	$ 0

Stock-Based Compensation (Details)	3 Months Ended
Stock-Based Compensation (Details)	Mar. 31, 2023 shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Granted (in shares)	4,694,691
Vested (in shares)	(540,238)
Nonvested ending balance (in shares)	5,835,016
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested beginning balance (in shares)	1,680,563

Stock-Based Compensation Summary of Stock-Based Compensation Expense Classification (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 1,645	$ 1,459
Research and development expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	831	704
Selling, general and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 814	$ 755

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Income Tax Disclosure [Abstract]
Federal income tax benefit	$ 0	$ 0
State income tax benefit	0	0
Provision for income taxes	$ 4	$ 23

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References + Details

Net Loss per Share - Summary of Basic and Diluted Net loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Numerator:
Net loss	$ (24,020)	$ (21,965)
Dividends, Paid-in-kind	0	(2,259)
Net loss attributable to common stockholders	$ (24,020)	$ (24,224)
Denominator:
Weighted average shares of common stock outstanding—basic	145,967,000	33,737,000
Weighted average shares of common stock outstanding—diluted	145,967,000	33,737,000
Net loss per share attributable to common stockholders—basic	$ (0.16)	$ (0.72)
Net loss per share attributable to common stockholders—diluted	$ (0.16)	$ (0.72)

- Definition

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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- Definition

Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

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+ Details

Net Loss per Share - Additional Information (Detail)	Mar. 31, 2023 $ / shares
Pre Funded Warrant \| Maximum
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Exercise Price (usd per share)	$ 0.01

Net Loss per Share - Schedule of Anti-dilutive Securities Excluded from Computation of Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - shares	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share (in shares)	96,386,089	13,438,974
Employee Stock Option
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share (in shares)	2,830,300	2,049,158
Restricted Stock Units (RSUs)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share (in shares)	5,835,016	1,940,788
Class C Warrant Liability
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share (in shares)	87,720,773	9,449,028